Abstract: Naphthenic acids(NAs) in the biotreated effluent of crude oil desalter wastewater treatment unit in an oil refinery were characterized by negative-ion ESI-MS. The results show that NAs are refractory organic matter for the biological treatment. The structures of NAs can be identified effectively under full scan, product-ion scan and neutral loss scan by negative-ion ESI tandem MS. Molecular weight of NAs range from 102 to 596 with carbon number C₅-C₄₁ and ring number 0-6. The congener distributions of NAs are quite similar to normal distribution pattern, and the highest intensity correspond to C₁₈.